1. Field of the Invention
The present invention relates to a drive circuit for an apparatus that, in an image capturing apparatus such as a digital camera, removes foreign substances adhering to the surface of an optical element disposed in front of an image sensor.
2. Description of the Related Art
Digital image capturing apparatuses as exemplified by digital still cameras, video cameras, and so on (hereinafter referred to simply as “cameras”) are quickly spreading due to their immediacy, high compatibility with personal computers, and so on. Such cameras obtain image data by photoelectrically converting an object image using an image sensor, and generally, in their most basic configurations, are composed of an image sensor, an imaging optical system, and an optical element such as a low-pass filter or the like disposed in front of the image sensor.
If, with the stated camera, a foreign substance such as dust or the like adheres to the optical element, that foreign substance itself will appear in the image, causing a drop in the image quality. For this reason, various techniques for removing foreign substances that have adhered to the optical element by causing the optical element to vibrate have been proposed, and are in recent years beginning to come into practical use (see Japanese Patent Laid-Open No. 2003-333391).
Incidentally, when removing foreign substances that have adhered to the optical element by causing the optical element to vibrate as mentioned above, a vibration apparatus that employs a piezoelectric element is generally used as the apparatus to cause the optical element to vibrate. When driving this piezoelectric element, the driving frequency is changed, in a continuous manner or in steps of approximately several tens of Hz, within a predetermined frequency range from several tens of kHz to several hundred kHz.
The reason for this is that a member that elicits a high Q-value is employed in order to achieve a large vibration when vibrating the optical element by exploiting the resonance phenomenon, and it is thus necessary to drive the piezoelectric element in the vicinity of a resonance frequency. At this time, the resonance frequency fluctuates due to reasons such as changes in temperature caused by driving and the like, and thus it is necessary, in order to achieve a high resonance, to alter the driving frequency in a continuous manner or in steps of approximately several tens of Hz as described above.
When generating a driving signal as described above, a voltage-controlled oscillator (abbreviated as “VCO” hereinafter) or the like is used in order to alter the frequency in a continuous manner, creating a frequency signal that is altered in a continuous manner by altering the input voltage. However, a VCO is an oscillator that employs an analog circuit, and thus there is a problem in that the oscillation frequency fluctuates due to individual variations in components, changes in temperature, or the like.
The following methods have been proposed as solutions. One such method generates a high-frequency and highly-accurate frequency signal using an element that is capable of generating an accurate frequency, such as a crystal oscillator, a ceramic oscillator, or the like, and causes a piezoelectric element to vibrate by performing frequency division based on that signal and generating a driving signal of an accurate frequency (see Japanese Patent Laid-Open No. 5-184169). Another such method causes a piezoelectric element to vibrate by generating a driving signal of an accurate frequency using a PLL synthesizer (see Japanese Patent Laid-Open No. 1-227671).
However, with the stated method that generates an accurate frequency signal with a high frequency and performs frequency division, the stated method that uses a PLL synthesizer, and so on, it is necessary to generate a high-frequency signal to serve as a base for altering the driving frequency in steps of approximately several tens of Hz. For example, assuming that the driving frequency is approximately 200 kHz, and the frequency is to be altered in steps of approximately 50 Hz, a signal of approximately 800 MHz is necessary. The generation of such a high-frequency signal is problematic in that it increases the amount of power consumed, increases the amount of unnecessary radiation, and requires the use of circuit components capable of operating at high speeds.